NEW YORK (CNN) -- The possibility of the republic of Montenegro becoming the next crisis in the Balkans was a topic of intense conversation among U.S. officials and world leaders on the sidelines of the United Nations Millennium Summit on Thursday.

President Clinton spent a good deal of time talking about Serbian aggression in Montenegro during his meeting with Russian President Vladimir Putin.

Deputy Secretary of State Strobe Talbot said the two leaders were concerned enough about the issue to remand further discussions to Albright and Russian Foreign Minister Ivanov, who held a working dinner on Wednesday night.

U.S. concerns about the possibility of Yugoslav President Slobodan Milosevic making a move on Montenegro have prompted the Clinton administration to beef up its presence in the region.

In recent weeks, the U.S. put its presence in the region on alert. Currently the amphibious assault ship U.S.S. Saipan, with some 1,900 marines from the 26th Marine Expeditionary Unit is in port at Trieste, Italy in the Northern Adriatic, about 30 minutes from Montenegro.

U.S. officials have planned military exercises in the area later this month. Officials say the joint manoeuvres with the Croatian military, planned for September 24, are meant to send a "strong warning to Milosevic" that a move against Montenegro would be unwise.

"No option has been taken off the table," said one senior State Department official.

The date September 24 is no accident, U.S. officials said. That is the date of recently announced federal elections in Serbia, called by Milosevic after he changed the Yugoslav constitution to enable him to remain in power for another term.

Montenegrin President Milo Djukanovic, a long-time rival of Milosevic who has advocated a redefinition of the republic's relationship with Serbia, has refused to participate in the election because he said they were illegally called and not free and fair.

U.S. officials say that while he recognizes the U.S. view that as many people as possible should support opposition candidates in the election, the Montenegrin leader gave his assurances he would support the opposition in "other ways that also exposes what Milosevic is doing."

"Djukanovic made clear that he would not participate for reasons that make sense," said another senior State Department official. "But he said the people of Montenegro are free to participate."

Administration officials and Balkan analysts all expect Milosevic to steal the election, as he will not be able to muster enough support to maintain power in a free and fair process.

He could also, say officials, invade Montenegro and create a "national emergency," which would enable him to cancel the election. Analysts fear he could render the Djukanovic regime illegitimate for not having participating in the elections, and institute a "bloodless coup" on the republic.

"This is a very dangerous time for Djukanovic," a U.S. official said.

U.S. and Montenegrin officials told CNN that Milosevic had been "tightening the screws" against Djukanovic and Montenegro.

In the past six months Milosevic has squeezed the Montenegrin economy with an internal economic embargo and has begun to "move military troops into areas along the border with Serbia."

Administration officials said they are worried that Milosevic will either arrest Djukanovic or invade Montenegro around the time of the September 24 Serbian election -- banking on the premise that the United States may be distracted by its own November presidential election.

Officials are closely watching for Milosevic to cut off Montenegrin access to Croatia and to attack the republic's independent media as indicators that a crisis is about to erupt.

"The tempo is up," said a senior administration official. "The threat is real."

Secretary of State Madeleine Albright has met several times with Djukanovic over the last several months. She met him again on Thursday morning to voice U.S. support for Montenegro.

"We showed our very strong support for Montenegro's integrity and its ability to run its own affairs," U.S. Ambassador Richard Holbrooke said after the breakfast meeting.

He added it would be "best" for Montenegro to maintain "self-rule and autonomy."

The Montenegrin president also met with European allies on the sidelines of the UN summit, including French President Jacques Chirac.

But despite the show of military might, U.S. officials acknowledge the possibility of the United States or NATO responding with force to a move by Milosevic on Montenegro is highly unlikely. "Nobody has the stomach for another Kosovo," said an official. "And there is no ethnic cleansing going on in Montenegro."

The U.S. fears that a move against Montenegro may embolden Milosevic to "stir the pot" in areas of northern Serbia, or to "muck around " again in Kosovo or the Republic of Serbska in Bosnia.

"He is restless," explained this official. "He has spent the last 13 years fermenting crisis. Crisis is his bread and butter and from which he derives the reasoning for his authoritarian regime."

Balkan analysts say that unless the Clinton administration is prepared to back these "messages" with actual force, Milosevic may see the United States as "indecisive."

"The whole reason for a threat is to show there is a line," said Ivo Daalder of the Brookings Institution. "And show that the U.S. will respond if he crosses it."